Electrostatic printing



Feb. 17, 1959 H. R I 2,874,063

ELECTROSTATIC PRINTING Filed March 23, 1953 'HERULD Ef EnEi E ATTORNEY United States Patent 2,874,063 ELECTROSTATIC PRINTING Harold G; Greig, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware This application is ,a continuation-in-part of my copending application, Serial No. 322,060, filed November 22, 1952, now abandoned, entitled Electrostatic Printing.

This invention relates broadly to electrostatic printing and more particularly to an improved developer mix for use in electrostatic printing, and methods for developing latent electrostatic images by means of such an improved mix. e

In one form of electrostatic printing, a latent image in the form of an electrostatic charge is produced upon a surface comprising a photoconductive insulating substance such as selenium. An explanation of the process and its principles may be found in an article entitled, Xerography, a new principle of photography and graphic reproductionby Shaifert and Oughton in the Journal of the Optical Society of America, December 1948, at page 991. A latent image so formed is developed by allowing a powder consisting of colored particles having a suitable electrostatic charge to come into contact with the image-bearing surface. The particles of the powder may be charged by any ordinary means, such as by contact with another charged surface, or they may be charged by means ofthe triboelectric effect by contact with carrier particles of another material separated from the powder in the triboelectri'c series. These colored particles are retained on portions of the surface by electrostatic forces in the charged area. The uncharged areas do not retain the particles but remain clear.

In the practice of electrostatic. printing it has been found desirable to use a carrier for the developer powder, Af carrier consists usually of relatively large particles of some'material separated in the triboelectric series from the material of the developer powder. The carrier and the powder are held together by the electrostatic forces'produced upon contact between them by the triboelectric effect. -Theusual method for developing a charged surface having alatent image is to flow the mixed carrier and powder by gravity across the surface so that the powder is-removed from the carrier by and retained on the charged areas of the surface.

The developer mix' comprising the carrier and powder must be brought into contact with the surface containing a latent image under light-excluding conditions. This requirement usually necessitates a relatively complex mechanical arrangement in that a tray containing the developer mix must be so arranged in respect of the surface to be developed that the tray and the surface are movable together.

An object of the invention is to providean improved method for electrostatic printing.

Another object of the invention is to provide an improved developer mix for use in electrostatic printing.

Another object of'theinvention is to provide an improved carrier" materialforuse in a developer mix for.

electrostaticprinting. 7

"Still anotherobject of the invention is to provide for usein' electrostatic printing an improved developer mix havingboth triboelectric andferromagnetic properties.

7 2,874,063 Patented Feb. 17, 1959 ice An important feature of the invention provides a developer mix comprising an ordinary developer powder and a ferromagnetic carrier material. In the practice of the invention, therefore, gravity flow need not be relied upon to effect contact between the developer mix and the charged surface. Instead, a magnetic field may be used for this purpose.

The invention will be more easily understood by reference to the following detailed description and to the drawing of which:

Figure 1 is a perspective view of a surface bearing a latent electrostatic image being developed according to a preferred embodiment of the invention.

Figure 2 is a fragmentary cross-sectional elevational view of the apparatus shown in Figure 1, looking in the direction indicated by the arrow A.

Figure 3 is an elevational, cross-sectional drawing of a surface bearing a latent electrostatic image being developed according to another embodiment of the invention.

Similar reference characters have been applied to similar elements throughout the drawing. f

The advantages which may be gained by control of the developer mix by magnetic forces are apparent when, for example, it is desired to develop an image on a moving cylindrical surface. The developer mix may be raised to develop the image on the underside of the cylinder. Likewise, magnetic carrier material alone, without developer powder, may be similarly applied to the surface to effect 'a cleaning of the surface, and the force of the cleaning action may be varied by adjusting the intensity of the magnetic field.

In a preferred embodiment of the present invention, a developer mix is provided comprising a developer powder and a carrier material consisting of alcoholized iron, that is, iron particles free from grease and other impurities soluble in alcohol. These iron particles are preferably of a relatively small size, being in their greatest dimension about .002" to .008". Satisfactory results are also obtained using a carrier consisting of iron particles in a somewhat wider range of sizes up to about .001" to .020".

Generally, any of the large number of developer powders known in the art is suitable for mixing with iron particles to form a developer mix according to the present invention. A developer powder above iron in the triboelectric series is generally suitable for developing a direct image upon a negatively charged surface or a reversed image upon a positively charged surface, and conversely a developer powder below iron in the tribeelectric series is suitable for developing a reversed image upon a like negatively charged surface or a direct image upon a positively charged surface. Examples of the developer powders of the first named class are powdered "second class is Vinsol resin (an extract from long leaf yellow pine tree stumps composed principally of an oxidized form of abietic acid and manufactured by Hercules Powder Company, 908 Market Street, Wilmington, Delaware) colored with a suitable dye such as azo oil black.

For purposes of illustration there is described herein the development of a direct image upon a negatively charged surface; however, it should be understood that by substituting asuitable different developer powder as described above there may be developed a reversed image upon such a surface. p

The term direct image when used in this application refers to a powder image developed upon a surface hearing a latent electrostatic'image in which powder particles -are caused to adhere to those portions of the surface bearing greater electrostatic charges than other portions. The term reversed image refers to the converse situation wherein powder particles are caused to adhere to portions of the surface bearing a lesser charge than other portions. The terms negative and positive are used in their ordinary electrical senses.

A preferred embodiment of the invention is illustrated by Figures 1 and 2 which show a sheet of negatively charged paper 2 bearing a latent electrostatic image on its surface indicated by the dotted lines 6 and supported by a suitable rigid body 4 which may be of brass. The paper 2 may conveniently be of the clectrophotographic type such as that described in my co-pending application, Serial No. 248,937 filed September 29, 1951, now abandoned and comprising a coating of relatively pure zinc oxide upon its surface. It may be charged by any convenient means such as a corona discharge to a potential of about -600 to -800 volts, and a latent image may be provided upon the surface by any convenient method such as photographic exposure or scribing.

A developer brush 10 is formed by dipping a bar magnet 12 into a mass of the developer mix as described above and comprising iron particles mixed with a carbon pigmented rosin powder in the proportion of about 50 to 1 by weight. The bar magnet may be in any convenient form, either electromagnetic or permanent, and of any desired shape. Most conveniently it may be a permanent magnet of a length sufiicient to extend completely across one dimension of the paper, and is polarized transversely as indicated by the arrow 14. When the magnet is withdrawn from the mass of developer mix, a quantity of the mix adheres by magnetic attraction'to the magnet forming a brush-like mass 16 along the magnet pole.

To 'develop the latent electrostatic image 6, the developer brush is lightly rubbed across the surface of the paper, causing particles of the pigmented resin to transfer from the brush to the charge image, and producing a direct powder image 8 which may be fixed to the surface by any conventional means such as heating.

In place of forming a developer brush by dipping a magnetinto a mass of developer mix, a quantity of developer mix may be placed upon the surface of the charged, image-bearing paper and the magnet then passed over the paper in close proximity thereto. Thepassage of the magnet gathers the developer mix into the form. ofa brush and simultaneously,the brush is swept across the surface as in the previous example, causing particles of the pigmented resin to adhere to the charge image. i

As additional electrostatic images are developed by the developer brush there results a depletion of developer powder in the mix composing the brush since a quantity of powder is deposited'upon each charged surface to form a powder image. The mix may be readily replenished by placing either developer powder or ,addi-' tional mix enriched in developer powder upon the charged surface to be developed. As the brush is passed over such a surface bearing veloper powder or mix, it picks up substantially all of the powder or mix from the discharged areas of the surface, leaving a powder layer coating and adhering to the charged areas.

Developer mixes comprising carrier particles different from those described above may also be satisfactorily employed in the practice of the invention according to the preferred embodiment. For example, the iron particles described may be coated with an organic insulating film such as shellac or varnish, or they may be coated with a metal fihn such as copper. Such coatings may be desirable in order to affect the triboelectric charge placed on the developer powder particles by contact with I the carrier particles; however, it has been found that iron particleswithout a coating provide a preferred car indiscriminately disposed de- 'the iron particles so printing powder compositions rier material, and that a developer mix according to the preferred embodiment produces powder images of superior quality having greater definition and density and a cleaner background than electrostatic powder images developed by other means.

The composition of the developer mix as described above is not critical. While a preferred mixture comprises about 2% by weight developer powder, the remainder being iron, satisfactory results may be obtained using a mixture comprising 1% to 6% by weight developer powder.

In a second embodiment of the invention, iron particles approximately spherical in shape and ranging in size from about 0.015" to 0.06" in diameter, but preferably more nearly 0.02", are coated with a polystyrene-cumar resin varnish from a toluene solution. The coated particles may be dried under a standard infrared heating lamp and under constant agitation sufficient to prevent them from sticking together. The dried coated particles are then used as a carrier for a carbon pigmented rosin powder, and an electrostatic image is developed by raising the mix from below to a surface bearing a latent electrostatic image. The carrier and powder mix may be repeatedly raised to the surface and released, or the mix maybe caused to travel acrossthe surface by changing the relative position of the magnetic field. Either procedure produces satisfactory results.

Figure 3 illustrates suitable apparatus for developing an electrostatic image in accordance with this embodiment of the invention. A conductive, non-ferrous metal plate 4 bearing a thin layer 3 of a photoconducting material, such as amorphous selenium, which has been electrically charged and photographically exposed, is placed over a tray 18 within the light-excluding flange 20. The tray bears a developer mix 22 comprising magnetic carrier particles and a developer powder. A magnet 12, which may be either a permanent or an electrically energized magnet, is moved along a track 24 with one pole 13 in constantly close proximity to the plate 4 throughout its extent of travel. The magnetic field, indicated by arrows net raises a portion 26 of the developer mix from the floor of the tray to the coated surface of the conductive plate forming a developer brush and holding it in contact with the surface. Developer powder is separated from the carrier material and held on the charged portions of the photoconducting 'surface by electrostatic forces, and when the magnet and developer brush have traversed the length of the plate the electrostatic image is completely developed. V

It has been found that the use of a polystyrene-cumar resin varnish coating upon the iron particles is not essential in the practice of this embodiment of the invention. The varnish coating is described because it is believed to afford a more complete electrical insulation between v coated. However, if this coating is omitted altogether and the bare'iron particles are combined with a suitable printing powder, a satisfactory image may be developed. Also,'any of a large number of other materials may be used to form an insulating coating upon the iron particles such as, for instance, shellac or a synthetic resin.

By selecting a suitable material with which to coat the magnetic carrier particles it is possible convenient- 1y to affect the polarity and intensity of the triboelectric tact with the carrier particles. This is desirable in order to permit the production of either a negative or a,

positive powder image upon the exposed printing surface.

It should be understood that rosin powder as described is not essential in the practice of the second embodiment of theinvention. Any of a large number of other may-be substituted therefor. v

15, emanating from the mag-,t

For example, satisfactoryresultsare obtained ina developer mix comprising iron particles and powders such as those described in United States Patent No. 2,297,691, issued to C. F. Carlson on October 6, 1942, and consisting of substances, such as gum copal, gum sandarac and sealing wax.

In selecting a developing powder for use in the practice of the invention, it is only necessary to select a material having triboelectric properties relative to the surface of the carrier material. If the powder and the carrier are widely separated from each other in the triboelectric series, it may be necessary to adjust upwards the electrostatic charge initially applied to the photoconducting surface. And, conversely, if the powder and the carrier are close to each other in the triboelectric series, it may be necessary to adjust downward the charge initially placed upon the surface. These adjustments may be necessary to ensure that the charged areas have sufiicient charge to cause the developer powder particles to leave the carrier particles and adhere to the charged surface, and that the exposed areas retain less than this amount of charge. Failure to adjust the initial charge on the surface in this manner may result in the production of no image, either because no developer powder adheres to the surface or because the powder adheres to all of the surface. However, it should be emphasized that the charge relationships do not appear to be critical and that it has seldom been found necessary to make any adjustment of the charge placed on the surface.

It should also be understood that the practice of the invention is not limited to the use of iron particles to form a carrier but that other ferromagnetic materials may also be used such as, for instance, magnetite.

It should be understood that the practice of the invention is not dependent on the type of insulating surface that bears the latent electrostatic image. Such images may be developed upon any suitable insulating surface, such as resin-coated paper, Bakelite or glass. For purposes of convenience the invention has been illustrated in connection with photoconductive surfaces which are more commonly used since they facilitate the creation of a latent electrostatic image by photographic means.

There have thus been described, for use in electrostatic printing, improved carrier materials, improved developer mixes, and improved methods employing magnetic means for developing latent electrostatic images.

What is claimed is:

1. A developer mix for developing latent electrostatic images, said mix comprising a dry mixture of loose, movable particles of electrostatically-attractable powder and separate magnetically-attracta-ble granules, said magnetically-attractable granules and powder particles having a triboelectric relationship of opposite polarity, the powder particles thereby being electrostatically-charged through triboelectric action by contact with the magnetically-attractable granules to adhere electrostatically to the surface of said magnetically-attractable granules and being attractable by charged areas of said latent electrostatic image when moved thereacross, and said magneticallyattractable granules being correspondingly electrostatically-charged to opposite polarity and thus adapted to attract said powder particles and to remove them from uncharged areas of said latent electrostatic image when moved thereacross, said magnetic granules being between 0.002 and 0.008 inch in their greatest dimension.

2. The developer mix of claim 1 in which said magnetic granules are iron particles and said developer powder constitutes 1% to 6% of the total weight of said developer mix.

3. The developer mix of claim 1 in which said magnetic granules are uncoated iron particles.

4. The developer mix of claim 1 in which said magnetic granules are iron particles, each particle having a continuous film coating thereon.

5. An electrostatic printing process for developing a 6. latent electrostatic image to yield an electrostatically-adhering image of electrostatically-attractable powder comprising magnetically transporting across said latent electrostatic image a dry mixture of loose, movable particles of electrostatically-attractable powder and separate magnetically-attractable granules, said magnetically-attractable granules and powder particles having a triboelectric relationship of opposite polarity, the powder particles thereby being electrostatically-charged through triboelectric action by contact with the magnetically-attractable granules to adhere electrostatically to the surface of the magnetically-attractable granules and being attractable by charged areas of said latent electrostatic image when moved thereacross, and said magnetically-attractable granules being correspondingly electrostatically-charged to opposite polarity and thus adapted to attract said charged powder particles and to remove them from uncharged areas of said latent electrostatic image when moved thereacross, said magnetically-attractable granules being of a size such that the gravitational force on each granule is less than the magnetic and electrostatic forces thereon.

6. The process of claim 5 including constituting said developer mix into a loose coherent mass with a magnetic field and then moving said magnetic field so as to contact said coherent mass with said latent electrostatic image.

7. The process of claim 5 including locating a magnet near said developer mix whereby a quantity of said developer mix is attracted as a loose coherent mass on said magnet, and then moving said magnet so as to contact said coherent mass with said latent electrostatic image.

8. The process of claim 5 including placing said latent electrostatic image over and near said developer mix and then applying a magnetic field to raise said developer mix into contact with said image.

9. The process of claim 5 including placing said latent electrostatic image over and near said developer mix, locating a magnet on the opposite side of said latent electrostatic image so that magnetic field lines extending through said latent electrostatic image and said developer mix attract said developer mix upwardly into contact with said latent electrostatic image.

10. An electrostatic printing process for developing a latent electrostatic image to yield an electrostatically-adhering image of electrostatically-attractable powder particles comprising constituting into a loose coherent mass with a magnetic field a quantity of developer mix comprising a dry mixture of loose movable particles of electrostatically-attractable powder and separate magneticallyattractable granules, said magneticallyattractable granules and said powder particles having a triboelectric relationship of opposite polarity, the powder particles thereby being electrostatically-charged through triboelectric action by contact with said magnetically-attractable granules to adhere electrostatically to the surface of said magnetically-attractable granules and being attractable by charged areas of said latent electrostatic image when moved thereacross, and said magnetically-attractable granules being correspondingly electrostatically-charged to opposite polarity and thus adapted to attract said charged powder particles and to remove them from uncharged areas of said latent electrostatic image when moved thereacross, said magnetically-attractable granules being uncoated iron particles between 0.002 and 0.008 inch in their greatest dimension, magnetically transporting across substantially said entire latent electrostatic image said quantity of developer mix while maintained as a loose coherent mass by said magnetic field, whereby developer powder particles from said developer mix adhere electrostatically to selected areas of said latent electrostatic image.

(References on following page) UNITED STATES PATENTS Sanders et a1. Dec. 22, 1896 Sutton et a1. July 11, 1905 Canton Oct. 27, 1931 Hay Mar. 31, 1936 Baermann Jan. 23, 1940 Osborne Apr. 29, 1941 Lehman Apr. 4, 1944 Carlson Sept. 12, 1944 Huebner 2 Sept. 24, 1946 Haskew Apr. 11, 1950 Walkup Nov. 18, 1952 Walkup et a1. May 12, 1953 FOREIGN PATENTS Great Britain Mar. 28, 1934 

5. AN ELECTROSTATIC PRINTING PROCESS FOR DEVELOPING A LATENT ELECTROSTATIC IMAGE TO YIELD AN ELECTROSTATICALLY-ADHERING IMAGE OF ELECTROSTATICALLY-ATTRACTABLE POWDER COMPRISING MAGNETICALLY TRANSPORTING ACROSS SAID LATENT ELECTROSTATIC IMAGE A DRY MIXTURE OF LOOSE, MOVABLE PARTICLES OF ELECTROSTATICALLY-ATTRACTABLE POWDER AND SEPARATE MAGNETICALLY-ATTRACTABLE GRANULES, SAID MAGNETICALLY-ATTRACTABLE GRANULES AND POWDER PARTICLES HAVING A TRIBOELECTRIC RELATIONSHIP OF OPPOSITE POLARITY, THE POWDER PARTICLES THEREBY BEING ELECTROSTATICALLY-CHARGED THROUGH TRIBOELECTRIC ACTION BY CONTACT WITH THE MAGNETICALLY-ATTRACTABLE GRANULES TO ADHERE ELECTROSTATICALLY TO THE SURFACE OF THE MAGNETICALLY-ATTRACTABLE GRANULES AND BEING ATTRACTABLE BY CHARGED AREAS OF SAID LATENT ELECTROSTATIC IMAGE WHEN MOVED THEREACROSS, AND SAID MAGNETICALLY-ATTRACTABLE GRANULES BEING CORRESPONDINGLY ELECTROSTATICALLY-CHARGED TO OPPOSITE POLARITY AND THUS ADAPTED TO ATTRACT SAID CHARGED POWDER PARTICLES AND TO REMOVE THEM FROM UNCHARGED AREAS OF SAID LATENT ELECTROSTATIC IMAGE WHEN MOVED THEREACROSS, SAID MAGNETICALLY-ATTRACTABLE GRANULES BEING OF A SIZE SUCH THAT THE GRAVITATIONAL FORCE ON EACH GRANULE IS LESS THAN THE MAGNETIC AND ELECTROSTATIC FORCES THEREON. 